1. Field of the Invention
The present invention relates to an improved method for the production of an activated vanadium/phosphorus mixed oxide catalyst, substantially free of chloride and volatile materials, from precursors thereof, the catalyst having special utility in the production of maleic anhydride.
2. Description of the Prior Art
Catalysts containing vanadium and phosphorus oxides have been used in the oxidation of 4-carbon atom hydrocarbons, such as n-butane, with molecular oxygen or oxygen containing gas to produce maleic anhydride. Conventional methods of preparing these catalysts involve reducing a pentavalent vanadium compound, and if desired, promoter element compounds under conditions which will provide or maintain vanadium in a valence state below +5 to form catalyst precursors which are recovered and calcined.
Hydrogen chloride has been used as a reducing agent for vanadium compounds where vanadium has a +5 valence. The use of gaseous HCl as a reducing agent is disclosed in U.S. Pat. No. 4,002,650 where the vanadium and phosphorus components are reacted in an aqueous solution. The use of gaseous HCl as a reducing agent for vanadium compounds such as V.sub.2 O.sub.5 is also described in U.S. Pat. No. 4,043,943 where the vanadium and phosphorus components are reacted in liquid organic medium.
U.S. Pat. No. 5,137,860 provides a comprehensive description of the prior art in this area. The patent shows the use of organic reducing agents as well as hydrogen chloride and teaches the use of activation procedures whereby the catalyst precursor without calcination is contacted at prescribed conditions with oxygen and steam mixtures and finally with a non-oxidizing steam atmosphere to produce an active catalyst.
U.S. Pat. No. 4,569,925 describes the preparation of vanadium/phosphorus mixed oxide catalysts by an organic solution method using anhydrous hydrogen chloride as an agent for the solubilization of the vanadium component, and teaches an activation procedure whereby the catalyst precursor is contacted not with air alone but with a mixture of air and a hydrocarbon such as methane, ethane, propane, butane and the like.
There are problems associated with the use of hydrogen chloride in the preparation of PVO catalysts. Even after calcination to prepare the catalyst, residual chloride remains in the catalyst. The chloride is generally removed during the catalyst activation period in the reactor, but chloride release from the solid catalyst into the reactor and the downstream equipment in the process is undesirable. The main problems are: possible equipment corrosion, product loss during chloride liberation and increased waste disposal. It would be advantageous to remove the chloride at the point of catalyst manufacture or at least before it is exposed to hydrocarbon feed in the reactor.